Conventional photo-sensors are typically made of semiconductor materials based on interfacial photovoltaic effects across a p-n junction or Schottky barrier. Such photo-sensors are known to possess numerous drawbacks associated with low photo-voltage, strong temperature dependency of the photocurrent, fast degradation under intense radiation and being expensive to fabricate. In view of these limitations, photo-sensors made of ferroelectrics thin films have been introduced as a promising alternative to their semiconductor counterparts. These sensors are typically based on bulk photovoltaic effect in the ferroelectrics and can be configured to produce large photo-voltage. In addition, ferroelectric oxide materials can be easily fabricated into thin films that possess excellent chemical and thermal stability. These attributes help to realize a robust, durable and low-cost photo-sensor which is highly desired for high intensity optical sensing and monitoring applications.
However, current ferroelectric thin-film photo-sensors are hindered by their low photocurrent magnitude. This is largely due to the high electrical resistivity as well as low optical absorption coefficient of ferroelectric materials. In any of these sensors, it is critical that the ferroelectric bulk is sufficiently exposed to the targeted radiation, so as to optimize the generation of photo-charges and elevate the sensor current output to photo-excitation. However, in current photo-sensors, photo-exposure on the ferroelectric bulk has not been maximized due to the presence of electrodes which impose shading effects on incident radiation.
Prior photo-sensors using ferroelectric thin films typically adopt a multi-layer structure comprising of a top metallic electrode, an electrically polarized ferroelectric thin film, a bottom metallic electrode and silicon (Si) substrate. Light is to pass through the top electrode first before reaching the ferroelectric film for the generation of bulk photovoltaic effect. Since the metallic surfaces of the electrodes are reflective in nature, the intensity of the radiation reaching the underlying ferroelectric bulk region is considerably attenuated. Such electrode shading effect gives rise to sub-optimal photo-excitation and severely impairs the performance of the sensor, including angular dependence and wavelength dependence. To mitigate the effects of electrodes shading, transparent conductive oxides (TCOs) have been deployed as top electrodes in ferroelectric sensors. Unfortunately, TCOs still have limited transparency to certain wavelengths (eg. ultraviolet) and possess much poorer electrical conductivities than metals. In addition, the top-bottom sandwich electrode configuration also has several other drawbacks, such as limited photovoltage magnitude, asymmetric interfacial energy barrier effect and degradation in prolonged intensive light irradiation.
Alternatively, a sensor configuration comprising of in-plane interdigitated metallic electrodes on a ferroelectric thin film may be adopted to improve optical exposure. In such a configuration, the polarization is aligned about in parallel with the surface of the ferroelectric thin film upon which both electrodes are usually deposited. Each electrode typically comprises of multiple fingers interdigitating those of the other electrode with a minute space gap in-between. With this in-plane configuration, the active areas in-between the electrodes are directly exposed to incident radiation. Research results have shown that the in-plane configuration of the ferroelectric photo-sensors have improved stability, durability and photovoltage magnitude. As there is no electrode shading effect on these areas, photon loss is reduced so as to achieve a more efficient photo-excitation with minimized angular dependence. However, the in-plane interdigitated electrodes configuration has yet to optimize optical exposure on the ferroelectric layer as a substantial area of the ferroelectric surface is still covered by the electrodes. These covered surfaces are subjected to sub-optimal optical exposure as a result of electrodes shading effects. In addition, the photovoltaic ferroelectric thin films may be exposed to moisture, contamination and mechanical scratches, which affect operation reliability and damages the ferroelectric layer.
There is thus a need to address the above drawbacks for existing photo-sensors.